This invention relates to a method of and apparatus for assembling a dynamoelectric machine, and more particularly to such method and apparatus in which the end shield or bearing support of an electric motor is secured to the stator core by a magnetic forming technique in such manner so as to establish a uniform air gap between the rotor body and the stator core.
Generally, an electric motor includes a stator comprising a core which is made up of a stack of laminations of suitable ferromagnetic sheet material. Each of the laminations is typically punched to have a central opening and a number of slots extending radially out from the central opening. The laminations are assembled in a stack, with the central openings of the laminations being coaxial so as to form a central bore through the stator core, and with the radial slots being in register with one another so as to form elongate slots extending through the core. Windings of suitable coils of magnet wire are inserted in the slots so as to constitute the windings of the motor. A rotor assembly, typically of squirrel cage construction, is received within the stator bore and is rotatable upon energization of the windings. The rotor assembly includes a rotor body made up of a stack of laminations of ferromagnetic material having longitudinal slots extending therethrough, generally proximate the outer periphery of the rotor body. Conductor bars extending through these longitudinal slots, and end rings interconnecting the ends of the conductor bars on the outside face of the rotor body are typically die cast-in-place on the rotor body. A rotor shaft is affixed to the rotor body such that the rotor body and shaft rotate as a unit. The rotor shaft is journaled in a suitable bearing which in turn is carried by an end shield or bearing support. The end shield may be of cast aluminum or the like, and is secured to the stator core in such manner that the bearing bore of the end shield is concentric or coaxial with respect to the centerline of the stator bore. In this manner, an air gap with a predetermined range of tolerances is provided between the outer surface of the rotor body and the inner portions of the stator core defining the stator bore.
The magnitude and efficiency with which a motor operates is dependent, to a large degree, on the air gap that is present between the outer surface of the rotor body and the inner surfaces of the stator core defining the stator bore. Generally, this air gap must be sufficient so as to prevent the rotor from physically touching the stator. Rotor strike will often lead to the failure of the motor. On the other hand, since the current induced within the rotor by the stator is inversely proportional to the distance from the stator core, a small, uniform air gap is desirable so as to maximize the operating efficiency of the motor.
Heretofore, in the assembly of electric motors and other dynamoelectric machines, a variety of techniques were utilized to align the rotor body with respect to the stator core so as to ensure a uniform air gap of minimum predetermined thickness. Typically, these motor assembly methods utilized shims which were placed between the outer surface of the rotor body and the stator core. These shims maintained the desired air gap while the end shield was affixed to the stator core. The shims were then removed. However, this oftentimes required precise machining of the portions of the end shield which engaged and which were secured to the stator and required that the end shield be adjustably moved relative to the stator to assume its proper position such that the rotor body would be journaled within the stator core within a desired degree of concentricity while maintaining the desired uniform air gap. The techniques were previously utilized to secure the end shield to the stator core in its desired position included the use of fasteners which could be secured after the desired position was attained, the use of adhesives to bond the end shield to the stator core, and the use of various magnetic forming techniques to clamp the end shield to the stator core by means of a separate clamping ring or the like.
This particular invention has reference to a so-called unit bearing motor, such as is described in detail in the co-assigned U.S. Pat. No. 4,209,722, to C. Theodore Peachee, Jr. This last-noted U.S. patent is herein incorporated by reference. In a unit bearing motor, typically only one end shield is provided which journals the rotor shaft, with the end shield being secured to the stator core. In this manner, the rotor is cantilever supported from the end shield bearing support. Of course, in other motor applications, two end shields are typically provided and both ends of the bearing are journaled. As shown in the above-noted U.S. Pat. No. 4,209,722, the end shield or bearing support has an inner surface thereon which is adapted to engage the outer surface and at least a portion of the outer periphery of one end face of the stator core. These surfaces of the end shield were typically machined to an accurate size so as to receive their respective portions of the stator core. The end shield may be secured to the stator core by means of a suitable adhesive or the like. However, this method required that the outside dimensions of the stator core be held within predetermined tolerances, and also required the machining of the inside surfaces of the outer portion of the end shield so as to ensure that the rotor journaled within the end shield was concentric within the stator bore.
As mentioned above, it is known that the bearing support or end shield of an electric motor may be secured to the stator core by assembling the several parts of the motor and placing them within a magnetic coil. Spacers were typically fitted between the stator and the rotor to assure uniformity of the air gap. After the parts were so asssembled and shimmed, a clamping ring surrounding the outer portions of the end shields and the stator core was swaged into place by the instantaneous energization of the magnetic coil which applied extreme compressive or swaging loads to the clamping ring which positively and instantaneously clamped the ends of the end shields to the outer surfaces of the stator. The primary advantage of such magnetic forming techniques in the manufacture of electric motors was that the use of axial tie bolts for securing the end shields to the stator core were eliminated which required precise concentricity between both the stator parts and the end shields. Also, the magnetic forming techniques enjoyed certain advantages over bonding the end shields to the stator core by fast-setting synthetic resins because split-second magnetic forming techniques were substantially faster. A discussion of magnetic pulse forming and, more particularly, the assembly of electric motors can be had by referring to the Tool And Manufacturing Engineer's Handbook, published by the Society of Mechanical Engineers, Third Edition, Copyright 1976, pages 17-22 through 17-30.
Other examples of the magnetic forming of the various parts of electric motors can be had by referring to U.S. Pat. Nos. 3,398,306 and 3,419,957, both of which disclose the use of a separate clamping ring which is magnetically formed around the outer surfaces of the stator and end shield to hold the end shield in position on the stator. U.S. Pat. No. 3,508,327 discloses an electric motor in which a conductive cylindrical outer housing is magnetically swaged onto the outer surface of the stator core in such manner as to form inwardly projecting indentations proximate the end faces of the stator core so as to lock the housing in place with respect to the stator core. U.S. Pat. No. 3,575,562 also, apparently, discloses a clamping ring which is magnetically formed to hold the end portions of the end shields of a motor on the stator core.
However, in spite of all of the known prior art methods and techniques of assembling electric motors, including the heretofore known magnetic pulse-forming techniques, it was still a problem as to how to provide accurate and consistent air gaps between the rotor body and the stator while the end shields were secured in place. Typically, most manufacturing techniques required the use of either very accurately machined parts so as to ensure concentricity, or the use of removable shims placed between the rotor body and the stator bore. Of course, the requirement of accurately machining increased the cost of the motor. The requirement of removable shims added complexity and time to the fabrication process.